One type of currently available article surveillance system utilizes an inductive magnetic field generator for deriving bursts of magnetic field energy having a predetermined carrier frequency, such as 60 KHz. The magnetic field energy derived from the generator is incident on a structure resembling a tuned circuit or a resistance-inductance-capacitance circuit carried by the article. Upon completion of the magnetic field burst from the generator, the structure re-radiates a magnetic field that is detected by a receiver, including a magnetic pick up coil. The re-radiated energy has a predetermined frequency that is transduced by a relatively high Q coil, tuned to the frequency emitted by the structure. The signal transduced by the coil is supplied to a relatively narrow band, high Q bandpass filter including resistances, capacitances, and operational amplifiers to resemble a tuned circuit. The tuned circuit thereby derives a narrow band signal which is supplied to appropriate processing circuitry for detecting the presence of the article containing the structure in the region between the transmitter and receiver coils.
A problem which has been encountered with this tuned prior art structure is that impulse magnetic fields have a tendency to cause ringing in the tuned coil and the bandpass filter. The ringing is likely to subsist for a relatively long time interval, approximating the length of time that the re-radiated field is derived from the structure on the article. Because magnetic field pulses occur on a rather frequent basis in response to power line surges and the like, there is a substantial tendency for erroneous indications of articles passing through the region under surveillance by the transmitter and receiver coils.
It is, accordingly, an object of the present invention to provide a new and improved article surveillance system.
Another object of the invention is to provide a new and improved article surveillance system that does not erroneously indicate the presence of an article.
A further object of the invention is to provide a new and improved inductive magnetic field article surveillance system that is relatively immune to magnetic field impulses.
A further object of the invention is to provide a new and improved inductive magnetic field article surveillance system having a narrow band width and which does not use tuned circuitry for a pick up coil or tuned circuitry responsive to such a coil.